Thanks to its capacity to ferment sugar into ethanol and carbon dioxide, the brewer's, wine and baker's yeast Saccharomyces cerevisiae has already been used for centuries for the production of bread, wine and beer. Apart from the production of heterologous proteins, S. cerevisiae is used in biotechnology primarily in the production of ethanol for industrial purposes. In numerous industries, ethanol is used as a starting substrate for syntheses. Due to the ever decreasing oil reserves, increasing oil prices and continuously rising global need for petrol, ethanol is increasingly becoming more important as a fuel alternative.
To allow for an economic and efficient production of bioethanol, the use of lignocellulose-containing biomass, such as e.g. straw, waste material from the timber industry and agriculture and the organic proportion of everyday domestic refuse, is a prime option as a starting substrate. On the one hand, it is very cheap and, on the other hand, available in great quantities. The three major components of lignocellulose are lignin, cellulose and hemicellulose. Hemicellulose, after cellulose the second most occurring polymer, is a highly branched heteropolymer. It consists of pentoses (L-arabinose, D-xylose), uronic acids (4-0-methyl-D-glucuronic acid, D-galacturonic acid) and hexoses (D-mannose, D-galactose, L-rhamnose, D-glucose) (see FIG. 1). Even though hemicellulose can be more easily hydrolysed than cellulose, it features the pentoses L-arabinose and D-xylose, which normally cannot be converted by the yeast S. cerevisiae. 
To be able to use pentoses for fermentations, they initially have to get into the cell via the plasma membrane. Although S. cerevisiae is not able to metabolize D-xylose, it can absorb it into the cell. However, S. cerevisiae does not possess any specific transporters. The transport takes place by means of the numerous hexose transporters. However, the affinity of the transporters for D-xylose is markedly lower than that for D-glucose (Kotter and Ciriacy, 1993). In yeasts, which can metabolize D-xylose, such as e.g. P. stipitis, C. shehatae or P. tannophilus (Du Preez et al., 1986), both unspecific low-affinity transporters, which transport D-glucose and specific high-affinity proton symporters only for D-xylose are present (Hahn-Hägerdahl et al., 2001).
Utilization of D-Xylose
Different bacteria, yeasts and fungi are able to metabolize xylose. In prokaryotes and eukaryotes, the metabolization of xylose mainly differs in the type of isomerization of xylose to xylulose. In prokaryotes, the conversion of xylose to xylulose takes place by means of the enzyme xylose isomerase (XI). In eukaryotes, xylose is mostly isomerized in two steps. Initially, xylose is reduced to xylitol by the NAD(P)H-dependent xylose reductase (XR) and further converted to xylulose by the NAD-dependent xylitol dehydrogenase (XDH). The subsequent phosphorylation reaction takes place in prokaryotes and eukaryotes by means of xylulokinase (XK).
The resulting intermediate xylulose-5-phosphate is an intermediate of the pentose phosphate pathway. The major part of the xylulose-5-phosphate enters the glycolysis in the form of fructose-6-phosphate and glyceraldehyde-3-phosphate and is therein further converted to pyruvate (Schaaff-Gerstenschläger and Miosga, 1997). Under fermentative conditions, the sugar is degraded further to ethanol by the pyruvate decarboxylase and the alcohol dehydrogenase. Under aerobic conditions, pyruvate can be oxidized to carbon dioxide in the citrate cycle by means of a series of reaction steps.
Utilization of D-Xylose in S. cerevisiae 
In papers from Kotter and Ciriacy (1993), a recombinant S. cerevisiae strain, which was able to metabolize D-xylose was constructed for the first time. For this, the genes of the yeast Pichia stipitis coding for D-xylose reductase (XYL1) and xylitol dehydrogenase (XYL2) were heterologously expressed in the yeast S. cerevisiae. In later works, the endogenous xylulokinase (XKS1) was additionally overexpressed, which improved the D-xylose absorption into the cell as well as its conversion to ethanol (Ho et al., 1998; Eliasson et al., 2000). Despite the achieved improvements, the main by-product of the xylose conversion under oxygen-limiting conditions was xylitol. This is attributed to an imbalance in the redox balance, which is caused by the reaction initially taking place in the metabolic pathway preferably using NADPH, however, the second reaction solely producing NADH (Hahn-Hägerdal et al., 2001). Under aerobic conditions, the NADH formed by the xylitol dehydrogenase can be regenerated to NAD via the respiratory chain. Under anaerobic conditions, NAD cannot be regenerated and accumulation of NADH in the cell results. Without the cofactor NAD, the xylitol dehydrogenase xylitol cannot be converted further to xylulose.
Although the xylose reductase used in the mentioned paper originates from P. stipitis, which is able to also use NADH as a cofactor, besides NADPH (Metzger and Hollenberg, 1995), the disruption of the xylose fermentation results under strict anaerobic conditions.
A solution to the problem was to introduce a redox-neutral metabolic pathway into S. cerevisiae. In prokaryotes, the conversion of xylose to xylulose takes place by means of the enzyme xylose isomerase (XI). For a complete conversion of D-xylose, only the gene XI would have to be expressed additionally as an endogenous xylulokinase is present. Although a xylose isomerase could be detected in some fungi (Tomoyeda and Horitsu, 1964; Vongsuvanglert and Tani, 1988; Banerjee et al., 1994; Rawat et al., 1996), only the xylose degradation via the enzymes xylose reductase and xylitol dehydrogenase has been shown in eukaryotes. Many efforts to heterologously express a xylose isomerase from different organisms failed (Gárdonyi and Hahn-Hägerdal, 1993). In the majority of cases, the enzymes were not functional in yeast or they were not synthesized to proteins (Sarthy et al., 1987; Amore et al., 1989; Moes et al., 1996). With high activity, only the xylose isomerase could be expressed in yeast from the obligatory anaerobic fungus Piromyces sp. E2 (Kyper et al., 2003). When heterologously overexpressing this eukaryotic xylose isomerase (Harhangi et al., 2003), S. cerevisiae was able to grow on xylose and also metabolize it under anaerobic conditions (Kuyper et al., 2003). However, further tests showed that the enzyme is strongly inhibited by xylitol, a product of the xylose conversion. Xylitol is formed unspecifically in yeast from xylose by means of aldose reductases.
U.S. Pat. No. 6,475,768 describes the use of a prokaryotic thermophilic xylose isomerase from Thermus thermophilus and variants of this, respectively, for the metabolization of xylose by yeasts. The optimal temperature for this enzyme or the variants is at a temperature (>70° C.), which is markedly higher than the temperature at which yeast grows and metabolizes (28-35° C.); however, yeast is inactive or dies off at temperatures above 40° C. However, at temperatures of about 30° C., the xylose isomerase from Thermus thermophilus and also the variants are virtually inactive. Thus, this enzyme and its variants do not permit the yeast to effectively metabolize xylose.
Therefore, a need exists in the prior art for pentose isomerases, particularly xylose isomerases allowing for an improved and more efficient pentose conversion, particularly xylose conversion.